There has been widely known a power converter that performs PWM control by comparing a modulated wave synchronized with an AC output voltage and a carrier wave consisting of a triangular wave, a saw-tooth wave, or the like and having a frequency higher than the frequency of the modulated wave. For example, Non Patent Literature 1 described below discloses a general technology concerning the PWM control. The PWM control includes a synchronous PWM mode (hereinafter simply referred to as “synchronous mode”) in which the modulated wave and the carrier wave are synchronized and an asynchronous PWM mode (hereinafter simply referred to as “asynchronous mode”) in which the carrier wave frequency is increased and control is performed without synchronizing the modulated wave and the carrier wave. For example, in a power converter for motor driving, it is widely known that, while an output voltage is increased as an output frequency increases, the power converter is shifted from the asynchronous mode to the synchronous mode and operated according to the increase in the output frequency.
Patent Literature 1 described below discloses a technology for gradually increasing the amplitude of a modulated wave according to an output voltage command value and, when a modulation factor at which the maximum amplitude of the modulated wave coincides with the maximum value of a carrier wave reaches π/4, shifting to an over-modulation mode in which a value of the modulated wave is always set larger than a value of the carrier wave for a period longer than one cycle of the carrier wave to stop switching. According to the technology of the patent literature, because the over-modulation mode is applied when the modulation factor is equal to or higher than π/4, it is possible to suppress occurrence of a narrow width pulse (hereinafter referred to as “narrow pulse”) around a peak value of an output voltage command in a state in which the modulation factor is equal to or higher than π/4.